An example of a control apparatus for a plant, which controls an air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine, is described in Japanese Patent Laid-open No. 11-73206. This control apparatus controls the air-fuel ratio using a self-tuning regulator having a parameter adjusting mechanism which functions as an identifier for identifying model parameters of the controlled object model.
In this control apparatus, a self-tuning correction coefficient KSTR is calculated by the self-tuning regulator according to the air-fuel ratio detected by an air-fuel ratio sensor provided in the exhaust system of the engine, and an amount of fuel to be supplied to the engine is controlled with the self-tuning correction coefficient KSTR.
When applying the above conventional control apparatus to a control of the internal combustion engine mounted on a vehicle, a problem that an emission amount of NOx increases upon deceleration of the vehicle caused by a quick return of the depressed accelerator pedal, or upon changing a gear position of a transmission of the vehicle, is confirmed.
When the accelerator pedal is quickly returned, an amount of intake air of the engine quickly decreases, and the fuel adhered to an inner wall of the intake pipe is supplied to the combustion chamber. Accordingly, the air-fuel ratio detected by the air-fuel ratio sensor indicates a spiky change to a richer air-fuel ratio (a change of quickly protruding and returning is hereinafter referred to as a “spike” or a “spiky change”). Therefore, in order to rapidly correct this change in the air-fuel ratio, the self-tuning regulator makes the self-tuning correction coefficient KSTR quickly decrease. As a result, the air-fuel ratio becomes over-lean immediately after the quick return of the accelerator pedal, which makes the emission amount of NOx increase.
FIGS. 14A to 14C respectively show changes in a detected equivalent ratio KACT, the self-tuning correction coefficient KSTR, a target equivalent ratio KCMD, the vehicle speed VP, and the NOx emission amount. The detected equivalent ratio KACT, shown by a thick line in FIG. 14A, is obtained by converting the air-fuel ratio detected by the air-fuel ratio sensor to an equivalent ratio. The target equivalent ratio KCMD, shown by a thin line in FIG. 14A, is obtained by converting a target air-fuel ratio to an equivalent ratio. The self-tuning correction coefficient KSTR is shown by a broken line in FIG. 14A. As shown in FIG. 14B, when a gear change is performed, a rich spike of the detected equivalent ratio KACT is generated, which causes the self-tuning correction coefficient KSTR to quickly change in the lean direction. Accordingly, a lean spike is generated immediately after the rich spike of the detected equivalent ratio KACT. As a result, the NOx emission amount temporarily increases as shown in FIG. 14C.
When applying the PID (proportional, integral, and differential) control to the air-fuel ratio control of the internal combustion engine, the above-described problem does not occur, since the response speed of the PID control is relatively slow. It is considered that the above-described problem is caused since the control by the self-tuning regulator has very high speed response characteristic.